1. Field of the Invention
The present invention relates generally to electroactive polymer structures and to devices that include electroactive polymers. The present invention also relates generally to methods for forming electroactive polymers, structures that include electroactive polymers, and devices that include electroactive polymers.
2. Description of Related Art
Generally, electroactive materials are defined as those materials which experience a change or otherwise “react” when influenced by an electric field or current. For example, some electroactive materials are piezoelectric and mechanically deform when exposed to an electric field. As such, currently available electroactive materials can be included in sensors, displays, actuators, batteries, and a variety of other devices.
Two families of polymeric electroactive materials have been developed: electronic electroactive polymers and ionic electroactive polymers. In ionic electroactive polymers, ion motion is associated with the electroactive effect, which can include volume change, color change, conductivity change, charge storage, and drug delivery. Members of the ionic electroactive polymer family include conjugated polymers, ionic polymer metal composites (IPMCs) (though the polymer membrane itself is not electroactive), gels (although not all gels respond electrically), and carbon nanotubes (although these are not polymers). In the present specification, the term “ionic electroactive polymer” may refer to any or all of these materials.
Typically, ionic electroactive polymers are formed as, and are used in the form of, films. Ionic electroactive polymer films typically have anisotropic physical and/or chemical structures and, as a result, possess anisotropic properties. For example, ionic electroactive polymer films may have anisotropic mechanical, electrical, and/or electrochemical properties, etc. FIG. 1 illustrates a representative electroactive polymer material 110 according to the prior art that is both polymeric and anisotropic.
As illustrated in the portion of FIG. 1 that illustrates a magnified portion of material 110, most of the polymer chains that make up material 110 have their backbones oriented along planes that are parallel to the top and bottom surfaces of material 110. In other words, there is relatively less cross-linking and/or chain orientation in the z-direction than in the x- and y-directions, in this example material.
Therefore, ions and/or other mass (e.g., solvent) may be transported more easily through material 110 in either the x-direction or the y-direction, since steric hindrance is relatively low, than in the z-direction. In other words, an ion moving through material 110 would run into fewer polymer chain backbones or crosslinks when moving in either the relatively “open” x- or y-directions than when moving in the z-direction.
As an alternative to the structure illustrated in FIG. 1, electroactive polymer material 110 may include monomer units that are oriented preferentially parallel to a substrate on which a film of material 110 is formed but with no preferential chain alignment. In the case of conjugated polymers with aromatic rings, the rings lie parallel to the surface of the material, which leads to transport anisotropy. Also, material 110 may be anisotropic if it includes crystalline regions with aromatic rings that are stacked face-to-face.
Anisotropic electroactive polymers, such as those found in material 110, commonly develop their anisotropy upon formation. For example, when an ionic electroactive polymer film is formed on a substrate, the surface properties of the substrate typically cause a majority of the backbones and/or aromatic rings of the polymer chains that make up the film to orient themselves in planes that are parallel to the substrate surface. Then, as more polymer chains are deposited to increase the thickness of the film, a majority of the backbones/rings of the additional polymer chains also orient themselves in such planes.
In some conjugated polymers, dopants have been found to facilitate anisotropy and/or ordering. For example, as illustrated in FIGS. 2A-2C, surfactant dopants 200 cause layering of polymer chains 210 within a polymer. Although not illustrated, aromatic dopants may also be used to promote stacking of conjugated polymer rings with the dopant rings and with each other. Further, liquid crystalline materials may also be used to order polymeric materials. In addition, in other ionic electroactive materials, additives can be used to facilitate anisotropy and/or ordering. Such additives may include surfactants and/or liquid crystals.
As illustrated in FIG. 2B, ion 220 may travel relatively easily in the xy-plane, which is relatively open. However, as illustrated in FIG. 2C, ion 220 has substantially more difficulty traveling in the yz-plane, due to steric hindrance. Therefore, the structures illustrated in FIGS. 2A-C have a higher degree of ionic conductivity in the xy-plane than in the yz-plane.
Those of skill in the art are aware that currently-available ionic electroactive polymers have a number of limitations that limit the types of applications in which they may be used. For example, it is well established that currently-available ionic electroactive polymer actuators suffer from having relatively long response times because their response is limited by the rate of ion and/or other mass transport through the material. Therefore, as the material becomes thicker, the speed of response of the device slows.
In order to minimize the response times of currently-available electroactive polymers, two methods have been employed. According to the first method, a high voltage is applied to the polymer or a high constant current density is used. However, high voltages and/or currents increase the likelihood that either the electroactive polymer, or an electrolyte contained therein, will break down. Therefore, response times can only be reduced to the extent that the electroactive polymer and/or accompanying electrolyte can support a high voltage or current.
Typically, an ionic electroactive polymer can sustain a high voltage or current only for a short time, until a certain charge has been consumed. Therefore, when using a high voltage or current to drive a device, the charge must be closely monitored. In addition, the speed of response does not keep increasing as the voltage or current is raised. Instead, the speed plateaus above a certain point. Therefore, using high voltages and/or currents to minimize response times has only limited utility.
The second commonly used method for decreasing response times of ionic electroactive polymers involves driving a device in resonance. Unfortunately, not all devices that include electroactive polymers can effectively be operated in such a manner. For example, resonance cannot be used to drive an actuator that controls an ON/OFF valve since, by definition, driving the actuator in resonance would cause the valve to “flap”, as opposed to remaining in either the ON or OFF position.